It is well known that metallic inks can be prepared by addition of aluminum foils to printing inks. Printed matter printed with a metallic ink, however, generates glittering parts in fine regions, resulting in insufficient specular gloss.
To solve such a problem, the applicant of the invention has proposed inks having the following composition in Japanese Patent Application No. H08-331465:
(1) An ink including 100 parts by weight of an aluminum foil, 15 to 200 parts by weight of a binder polymer, and 600 to 3000 parts by weight of a solvent, wherein the aluminum foil contains aluminum foils with thicknesses of 0.5 μm or less and foil surface areas of 20 μM2 to 2000 μM2 in an amount of 75% or more; and
(2) An ink including 100 parts by weight of an aluminum foil, 15 to 2000 parts by weight of a binder polymer, and 600 to 3000 parts by weight of a solvent, wherein the aluminum foil contains aluminum foils with thicknesses of 0.3 μm or less and foil surface areas of 20 μm2 to 2000 μm2 in an amount of 75% or more:
and, in the above compositions, the viscosity of the inks is preferred in the range of 50 to 1000 cps (measured at 20° C. with a BM-type rotational viscometer). Such compositions and viscosity range are disclosed in Japanese Unexamined Patent Publication No. H10-158561 (Patent Document 1) and U.S. Pat. No. 3,151,606.
The above-mentioned prior invention (hereinafter, referred to as the “applicant's prior invention”) can provide significantly high specular gloss as well as, within a preferred viscosity range, prevent sagging in terms of keeping contours of printed regions, and also realize highly qualified specular prints.
Further improvements are, however, expected with the applicant's prior invention in regard to enhancement of the specular gloss by further selecting a thickness of aluminum foils, and to prevent sagging by further defining the lower limit value of the viscosity for keeping not only contours in printed images but also clearances between thin lines (specifically, a clearance of 0.15 mm).
On the other hand, Japanese Examined Patent Publication No. S62-45905 (Patent Document 2) discloses inks which contain aluminum foils with thicknesses of 0.035 to 0.045 μm and foil surface areas of 20 μm2 to 2000 μm2 in an amount of 90% (Example 8), and are composed of 100 parts by weight of the aluminum foils, 20 parts by weight of a binder polymer of nitrocellulose (Example 9), and 2770 parts by weight of a solvent selected from ethylcellosolve, MEK, methylcellosolve, and toluene (Examples 6, 8, and 9).
However, if the ink composition disclosed in the above-described Japanese Examined Patent Publication is applied, test results obtained by the inventors of the present invention according to such compositions have shown that the ink obtained had a viscosity of 17 cps or less (measured at 20° C. with a BM-type rotational viscometer) by causing significant sagging. With regard to this point, improvements are necessary to solve this problem.